The present invention relates to a half-tone processing method which expresses appropriately a picture image having a continuous tone using a limited level value.
A half-tone processing method is to adjust a change of a tone smoothly by expressing every pixel value having continuous values with variously sized dots or with variable diffusion levels of the same sized dots. Since such a half-tone processing method can express the continuous tone with a limited level, it reduces the cost and the complexity of image expression.
In the dithering method which is one of the conventional half-tone processing methods, an output is supplied if an input value is greater than a predetermined value (threshold value) and otherwise an output is not supplied. Since such a dithering method does not consider human's visual sense, half-tone expression is hard to accomplish.
Another example of conventional half-tone processing methods is an ordered dithering. The ordered dithering which expresses the intensity of dots which are mutually adjacent or remote, obtains a dither matrix in consideration of the spatial distribution of dots, compares the respective threshold values and the pixel values and outputs the result as a half-tone processed pixel value. Such a method shows a smoother output than that of dithering method, but produces a Moire phenomenon, which shows a sense of spreading in an expressed image.
Still another example of conventional half-tone processing methods is error diffusion. The error diffusion method computes an error between an original pixel value and a half-tone processed pixel value, which is generated as a compared result of an original pixel value and a predetermined threshold value (in general, the arithmetic mean values of adjacent pixels of an objective pixel), then multiplies the error value by a weight considering a spatial relationship between the objective pixel and the adjacent pixels thereof and diffuses the multiplied error value. The error diffusion method can eliminate the Moire phenomenon, but produces a texture pattern in which a geometrical pattern is repeated. The texture pattern may offend the human's visual sense by repeating a specific pattern.